//https://acm.hdu.edu.cn/showproblem.php?pid=5979
//简单几何题
//在以原点为圆心的球上取几个点，告诉你每对连续的点与原点连线的夹角，求每对点连线构成的三角形总面积
#include<bits/stdc++.h>
using namespace std;
const double PI=acos(-1);
int main(){
    int n;
    double r;
    while(cin>>n>>r){
        double ans=0,x;
        while(n--){
            scanf("%lf",&x);
            ans+=sin(x/360*PI*2)*r*r/2;
        }
        printf("%.3lf\n",ans);
    }
}